You don't have to be a genius to understand that buying in bulk can save you money. But you don't have to be a genius, either, to realise that buying more can lead to consuming more.

Since opening in six sites across Australia in 2009, US-based company Costco has proven a hit with Australian consumers thanks to its 'bigger is better' shopping model. But health experts warn that supersized bargains are supersizing our waistlines.

Good for economic growth, not so good for you

Garry Egger, a Professor of Lifestyle Medicine at Southern Cross University and co-author of Planet Obesity: How we are eating ourselves and the planet to death, told CHOICE that stores like Costco are adding to what he calls a pattern of "consumption for economic growth". Offering more food at cheaper prices encourages greater consumption, Egger says, which is good for business all-round. "We're encouraged to buy more than we need through the system, and we've passed the sweet spot to the point where it's actually causing disease."

Egger argues that the food industry's push for greater consumption, especially in the western world, is the major force behind Australia's obesity epidemic. He believes stores selling in bulk are putting a burden on health systems that increasingly have to deal with weight-related disease. "It's a cost saving for them, but it's a cost increase to the community as a whole."

Buying more = eating more

Lisa Renn, an accredited dietician and member of the Dietitians Association of Australia (DAA), agrees that buying in bulk encourages unhealthy eating habits. "I see people on a daily basis who tell me that if there is food in the house in large quantities, they're going to eat it. What we're seeing with these bulk purchases is that people are buying the 24-can slabs of Coke, they're buying bigger packets of chips and bigger blocks of chocolate."

But Renn believes it's not just the food providers who should do more to encourage healthy consumption. "DAA supports a collaborative approach, which means involving government, the food industry, the media, marketers and consumer groups. But the buck stops also at the individual."

Costco declined to answer questions about any connection between bulk buying and unhealthy eating. "Costco is a wholesaler and our bulk packaging is designed to offer the best value to our wholesale members," the company said in a written statement.

Is bulk-buying worth it?

Comparing prices and groceries on a like-for-like basis with supermarkets such as Coles and Woolworths is difficult due to the huge difference in package volumes. Unit pricing would help in some cases, but you still need to do some calculations to determine just how much you would save buying a 20-litre tin of olive oil, for example, compared to the 750ml at your local store.

At the end of the day, you need to look at your own buying and eating habits to decide whether the cost benefit of buying in bulk is worth the potential health problems it could cause.

Costco vs Campbells

Costo and Campbells Cash 'n' Carry appear to be two very similar businesses, but a visit to their stores reveals some noticeable differences.

Campbells Cash 'n' Carry has a distinctive business look and feel, with less marketing, less effective lighting to highlight products on sale and a range of standard brands and products that most of us would recognise. The lighting in a Costco store is similar to a shopping mall and there may be more choices available for each particular product line. But perhaps the most significant factor highlighting the difference between the two stores is the trading hours.

Costco's operating hours are 10.00am–8.30pm during the week and extended trading hours on Saturday and Sunday. These hours are ideal for the shopper to duck out during a lunch break or take time after work. If you want to brave the shopping experience on a weekend, you may find Costco to be as busy as any suburban shopping mall.

Campbells Cash 'n' Carry operating hours are mostly 7.30am–4.30pm during the week, 8.00am–1.00pm on Saturday and closed on Sunday. These hours target businesses, not household shoppers.

Both stores require a membership card to purchase goods. Anyone can sign up to get savings at Costco if they pay a $60 membership fee. For Cash 'n' Carry the membership is free, but you need an Australian Business Number (ABN) to join. This may appear to be a significant restriction until you realise that a friend of a friend probably has a card you can use.